Cathode material for use in lithium secondary battery and manufacturing method thereof

ABSTRACT

A cathode material for use in a lithium secondary battery of excellent thermal stability contributing to the improvement of safety of the battery and having large discharging capacity, as well as a method of manufacturing the cathode material for use in the lithium secondary battery, based on the improved method for measuring the thermal stability of the cathode material, the cathode material comprising a compound represented by the chemical formula: Li x Ni y Co z M m O 2  in which the material is powdery and the BET specific surface area is 0.8 m 2 /g or less, M in the chemical formula represents one or more of element selected from Ba, Sr and B, and x, y, z and m are, respectively, 0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5 and 0.0005≦m≦0.02.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns a cathode material for a lithium secondarybattery and, more in particular, it relates to a cathode material for alithium secondary battery in which an alkaline earth metal or the likeis blended in a composite lithium nickelate-cobaltate, as well as amanufacturing method thereof.

2. Related Art

Lithium nickelate as a cathode material for lithium/ secondary batterieshas a feature that the discharging capacity is large. However, whencharge/discharge are repeated, lithium nickelate in which lithium ionsare deintercalated tends to undergo phase change into a state with nolithium disorder. This trend is remarkable as the working temperature ofthe battery is higher.

Since the phase change is an irreversible reaction, the absolute amountof the lithium nickelate that contributes as the cathode material forthe cathode active material results in lowering the dischargingcapacity. Further, evolved oxygen tends to react with the electrolyteconstituting the battery and, accordingly, it may be even lead to a riskof ignition or bursting of the battery when the working temperature ishigh.

In order to solve the problem, Japanese Patent Laid-Open No. 23629/2001,for example, proposes a method of evaluating the thermal stability ofthe lithium secondary battery by using lithium nickelate as the cathodematerial to form a lithium secondary battery, conductingcharging/discharging operation to the secondary battery thereby causingdeintercalation of lithium ions from the lithium nickelate and thenconducting thermogravimetry of the lithium nickelate. Further, as apreferred active material for use in the cathode of the lithium ionsecondary battery evaluated by the method described above, it disclosesa composition containing Co at a ratio of 0.05 to 0.3 mol based on onemol of Ni and further containing one or more of elements selected fromB, Al, Mg, Ca, Sr, Ba, Fe, Ti, Zr, Y, La and Ce by 0.001 to 0.1 mol intotal.

However, it has been found as a result of the study made by the presentinventors that the safety of the battery can not be evaluatedsatisfactorily by the existing method described above. An actual exampleis shown below. It is considered that oxygen gas evolves at a hightemperature region near 170° C. or higher on the cathode in a statewhere the battery is charged, and it causes concerns that the evolvedoxygen and the electrolyte constituting the battery may react. In a caseof evaluating the safety of the battery whose cathode material isexposed to a high temperature by measuring the decrease of the weight ofthe cathode material in a temperature change from 200 to 300° C. bythermogravimetry, although swelling or ignition of the battery can beestimated based on the decomposition gas from the cathode or theelectrolyte, it can not be estimated that such an abnormality will occurin the battery by the starting of reaction between the oxygen evolvingfrom the cathode and the electrolyte. As described above, the safety ofthe battery can not be recognized well by the existing method describedabove.

SUMMARY OF THE INVENTION

This invention intends to propose a cathode material excellent inthermal stability that contributes to the improvement of the safety ofthe battery, as well as a manufacturing method thereof.

The cathode material for use in the lithium secondary battery accordingto this invention comprises a compound represented by the chemicalformula: Li_(x)Ni_(y)CO_(z)M_(m)O₂ and a lithium secondary batteryhaving high thermal stability and large discharging capacity can beobtained by using the material. In the chemical formula described above,M is one or more of elements selected from Ba, Sr and B, x, y, z and mrepresent values for the molar ratio of each element and they are,respectively, 0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5 and 0.0005≦m≦0.02. TheBET specific surface area of the cathode material is 0.8 m²/g or less,and more preferably, less than 0.5 m²/g.

The compound represented by the chemical formulaLi_(x)Ni_(y)Co_(z)M_(m)O₂ is a compound represented by the chemicalformula Li_(a)Ni_(b)Co_(c)M_(n)O₂ in the charged state, and the DTGincrease start temperature is preferably 215° C. or higher. In thechemical formula described above, M is one or more of elements selectedfrom Ba, Sr and B, and x, y, z, m, a, b, c and n are a values for themolar ratio of each element and they are, respectively, 0.9≦x≦1.1,0.5≦y≦0.95, 0.05≦z≦0.5, 0.0005<m<0.02, 0.2≦a≦0.4, 0.5≦b≦0.95,0.05≦c≦0.5, and 0.0005≦n≦0.02. The DTG increase start temperature ismore preferably 230° C. or higher.

The tap density of the cathode material is preferably 1.5 g/cm³ or more,in which the amount of the cathode material filled in the battery can beincreased to further increase the charging/discharging capacity per unitvolume of the battery.

The cathode material for use in the lithium secondary battery can beproduced by mixing a compound represented by Ni_(y)Co_(z)(OH)₂, alithium salt and a salt containing the element M, heating andpulverizing them to obtain a cathode material for use in the lithiumsecondary battery represented by the chemical formulaLi_(x)Ni_(y)CO_(z)M_(m)O₂ in which the compound represented byNi_(y)Co_(z)(OH)² has a tap density of 1.8 g/cm³ or more and an averageparticle diameter of 5 to 20 μm measured by laser diffraction method.The shape of the powdery particles of the compound represented byNi_(y)Co_(z)(OH)² is preferably spherical. In this case, the heating ispracticed preferably by successively conducting a preliminary heating at300 to 500° C. for 2 to 6 hrs in an oxygen atmosphere, a temperatureelevation step at a rate of 5 to 30° C./min after the preliminaryheating and a final heating step at 650 to 900° C. for 2 to 30 hrssucceeding the temperature elevation step. In this case, the element Mis one or more elements selected from Ba, Sr and B, and x, y, z and mare, respectively, 0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5 and 0.0005≦m≦0.02.

Further, this invention provides a lithium secondary battery in whichthe active cathode material comprises partially or entirely the cathodematerial described above.

According to this cathode material for the lithium secondary battery ofthis invention, the safety of the secondary battery can be improved andconcern about ignition or bursting of the battery can be avoided even ina case where the secondary battery is exposed to a high temperaturestate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the change of DTG when the temperature ofvarious kinds of cathode materials for use in lithium secondarybatteries in a charged state in an argon atmosphere under the conditionof 10° C./min; and

FIG. 2 is a graph showing a relation between a BET specific surface areaand a DTG increase start temperature of a cathode material for use in alithium secondary battery.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors have found that a difference is observed for theDTG curves (Derivative Thermogravimetry: primary differentiation curvesregarding the temperature of thermogravimetry curves) for the chargedcathode material between each of the materials, and a cathode materialwhich starts to DTG change at higher temperature is characteristic ofexcellent thermostability. Further, it has also been found to beimportant that the cathode material has a determined composition and canmaintain a small specific surface area of the material for providingsuch a characteristic. This invention has been accomplished based on thefindings described above.

FIG. 1 shows DTG curves of the cathode materials for lithium secondarybattery of Nos. 3, 6, 9, 11 and 14 shown in Table 1. As shown in thegraph, DTG of the cathode materials starts to increase at a temperatureabout from 190° C. and has a peak between 220 to 290° C. The presentinventors have measured DTG for a number of cathode materials andinvestigated the relation between the DTG increase start temperature andthe safety of the battery.

TABLE 2 Test result for DTG increase Initial discharging Hightemperature Composition start temperature capacity storage left at Nailpenetration No. Li_(x)Ni_(y)Co_(z)M_(m)O₂ (° C.) (mAh/g) 150° C., 5 hrtest 1 Li_(1.02)Ni_(0.9)Co_(0.1)Ba_(0.001)O₂ 219 194 No swelling, noIgnition in some ignition battery 2Li_(1.02)Ni_(0.7)Co_(0.3)Ba_(0.001)O₂ 230 180 No swelling, no Noignition ignition 3 Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ 227 186 Noswelling, no Ignition in some ignition battery 4Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ 229 185 No swelling, no Ignitionin some ignition battery 5 Li_(0.96)Ni_(0.8)Co_(0.2)Sr_(0.007)O₂ 230 181No swelling, no No ignition ignition 6Li_(0.99)Ni_(0.8)Co_(0.2)Ba_(0.002)Sr_(0.001)O₂ 230 185 No swelling, noNo ignition ignition 7 Li_(1.06)Ni_(0.8)Co_(0.2)B_(0.02)O₂ 223 190 Noswelling, no Ignition in some ignition battery 8Li_(1.01)Ni_(0.8)Co_(0.2)Sr_(0.002)O₂ 224 184 No swelling, no Ignitionin some ignition battery 9 Li_(1.02)Ni_(0.7)Co_(0.3)Ba_(0.005)O₂ 248 184No swelling, no No ignition ignition 10 Li_(1.06)Ni_(0.8Co)_(0.2)Sr_(0.005)O₂ 225 185 No swelling, no Ignition in some ignitionbattery 11 Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ 201 177 Swelling insome Ignition in all battery battery 12Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ 203 175 Swelling in some Ignitionin all battery battery 13 Li_(0.96)Ni_(0.8)Co_(0.2)Ba_(0.03)O₂ 220 150No swelling, no Ignition in some ignition battery 14Li_(0.96)Ni_(0.8)Co_(0.2)O₂ 190 190 Swelling or Ignition in all burstingin all battery battery 15 Li_(1.02)Ni_(0.7)Co_(0.3)Ba_(0.005)O₂ 203 169Swelling in some Ignition in all battery battery 16Li_(1.06)Ni_(0.8)Co_(0.2)Sr_(0.001)O₂ 207 170 Swelling in some Ignitionin all battery battery

The measuring method for DTG and initial discharging capacity wasconducted according to the following procedures. After addingN-methyl-2-pyrrolidone to 90 mass % of a cathode material powder, 5 mass% of acetylene black and 5 mass % of polyvinylidene fluoride andthoroughly kneading them, it was coated to about 150 μm thickness on analuminum substratum, and, after pressing at about 200 kg/cm², it waspunched into a disk of 14 mm diameter. The disk was vacuum-dried at 150°C. for 15 hrs and used for the cathode. A lithium metal sheet was usedfor the anode and a porous polypropylene film (CELGARD #2400, trade nameof products manufactured by Celgard Inc.) was used for the separator.Further, 1 mol of LiClO₄ was dissolved in 1 liter of a mixed solution ata 1:1 by volumic ratio of ethylene carbonate (EC) and dimethyl carbonate(DMC) to prepare a non-aqueous electrolyte.

They were used and assembled into a test cell in an argon-substitutedglobe box and charging/discharging were conducted with a current densityat a constant value of 1 mA/cm² and under a voltage within a range from2.75 to 4.2 V to measure the initial discharging capacity. Afterconducting the charging/discharging as described above, the cathode wastaken out of the test cell in the 4.2 V charged state, the cathodematerial powder was peeled from the aluminum substratum, which wascleaned with dimethyl carbonate and vacuum-dried at 100° C. When themolar ratio for each of the elements of the cathode materialLi_(a)Ni_(b)CO_(c)M_(n)O₂ for a lithium secondary battery thus obtainedwas calculated by chemical analysis, the molar ratio a for Li was withina range from 0.2 to 0.4 mol based on the total amount of Ni and Co. Thispowder was heated in an argon atmosphere at a rate of 10° C./min byusing a thermogravimetric apparatus to measure the DTG and determine theDTG increase start temperature. The DTG increase start temperature meansthe temperature at which DTG exceeds 0.015%/° C. The cathode material orthe compound used for the material when the cathode was taken out of thetest cell in the charged state of the battery is referred to simply asthe “cathode material” or the “compound in the charged state” in thisspecification.

The safety test for the battery was conducted as described below.

-   (1) N-methyl-2-pyrrolidone was added to 90 mass % of lithium    nickelate composite oxide, 5 mass % of carbon black and 5 mass % of    polyvinylidene fluoride to form a paste, which was coated and dried    on an aluminum foil to prepare a cathode.-   (2) A paste comprising a graphite powder and N-methyl-2-pyrrolidone    was coated and baked on a copper foil to prepare an anode.-   (3) A separator was put between the cathode and the negative    electrode, and one mol of LiPF₆ dissolved in 1 liter of a mixed    solution at 1:1 volumic ratio of ethylene carbonate (EC) and diethyl    carbonate (DEC) was used as the electrolyte, to prepare a    cylindrical lithium ion secondary battery of 18 mm diameter and 65    mm length.-   (4) Cylindrical batteries each having an identical cathode material    were manufactured by the number of 10 and each five of them was    subjected to a high temperature storage test and nail penetration    test.-   (5) In the high temperature storage test, deformation or bursting,    or absence or presence of ignition accompanying bursting of the    batteries was investigated in a case of storage in a 4.2 V charged    state at 150 C. for 5 hrs in an atmospheric air.-   (6) In the nail penetration test, a nail of 2.5 mm diameter was    penetrated through the battery in a 4.4 V charged state in    atmospheric air to investigate the absence or presence of ignition    from the batteries. This is a severer test than the high temperature    storage test as a test regarding the safety of the battery at high    temperature.

As a result, it has been found that there is a correlation between theDTG increase start temperature of the cathode material and the safety ofthe battery according to the evaluation standard described above asshown in Table 1. The battery having higher DTG increase starttemperature is excellent in the safety of the battery. Since DTG isconsidered as an index of the thermal decomposition rate of the cathodematerial in the temperature elevation process, it is estimated thatthose of higher increase start temperature provided improvement for thesafety of the battery. As can be seen from Table 1, the results of thehigh temperature storage test for battery are satisfactory in a case ofthe cathode material of higher DTG increase start temperature with thetemperature of about 215° C. being the boundary. Further, in the nailpenetration test, the results are different with respect to the DTGincrease start temperature at 215 ° C. as the boundary and the resultsare satisfactory at 215° C. or higher. Further, in those having the DTGincrease start temperature of 230° C. or higher, the results areextremely excellent. As described above, it can be estimated that thesafety of the battery is improved more when the DTG increase starttemperature is higher.

FIG. 2 is a graph showing the relation between the BET specific surfacearea and the DTG increase start temperature in a charged state of acathode material for a lithium secondary battery. As shown in the graph,it can be seen that there is a close relation between the DTG increasestart temperature of the cathode material for the lithium secondarybattery of this type and the specific surface area thereof. When the BETspecific surface area is 0.8 m²/g or less, the DTG increase starttemperature is 215° C. or higher. In the chemical formula describedabove, M is one or more of elements selected from Ba, Sr and B, and x,y, z and m are, respectively, 0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0 5, and0.0005≦m≦0.02. The specific surface area was measured by the BET methodof determining based on the adsorption amount of a nitrogen gas. Thereason for defining the chemical formula Li_(x)Ni_(y)CO_(z)M_(mO) ₂ isexplained as below. At first, in this invention, the lithium nickelateof a large discharging capacity is used as a base. LiNiO₂ itself hashigh discharging capacity among cathode materials. However, it has onthe other hand, a problem in view of safety. Then, 0.05 to 0.5 mol of Niis substituted for Co to enhance the thermal stability. Improvement ofthe thermal stability is observed by Co at 0.05 mol or more and, on theother hand, discharging capacity is lowered if it is 0.5 mol or more.Further, B, Sr or Ba is blended at a ratio of 0.0005 to 0.02 mol basedon the total amount of Ni and Co to improve the thermal stability andobtain a sufficient discharging capacity. The improvement for thethermal stability by the element is insufficient when it is less than0.0005 mol, whereas the discharging capacity is lowered when it is 0.02mol or more. When Li is insufficient, this results in a crystalstructure with high lithium disorder and lowers the dischargingcapacity. Further if Li is excessive, it forms hydrates or carbonates,which are gelled upon manufacture of the electrode. A satisfactory pastestate is not obtained and worsens the handlability. Accordingly, Li isdefined within a range from 0.9 to 1.1 mol based on the total amount ofNi and Co.

In the invention described above, for increasing the dischargingcapacity per unit volume of the battery by increasing the amount of thecathode material to be filled in the battery, the tap density ispreferably 1.5 g/cm³ or more. The tap density was measured by using apowder tester manufactured by Hosokawa Micron Co. and applying tappingfor 200 times by using a 100 ml vessel for use in tap densitymeasurement.

The cathode material for the lithium secondary battery described abovecan be manufactured as described below. At first, a high densityNi_(y)Co_(z)(OH)₂ in which the ratio of Co is controlled to 0.05 to 0.5mol based on the total amount of Ni and Co as the starting material isprepared. Upon preparation thereof Ni_(y)Co_(z)(OH)₂ is prepared, forexample, by a wet solution synthesis method in which the averageparticle diameter is controlled to 5 - 20 μm and the tap density iscontrolled to be 1.8 g/cm³ or more. Each of the particles is desirably aspherical shape. In a case of synthesizing lithium - nickelate compositeoxide, since the shape of Ni_(y)Co_(z)(OH)₂ as the starting materialremains as it is, it is desirable to prepare the starting material asdescribed above to obtain a cathode material of a small specific surfacearea and a high tap density according to this invention. After mixingthe spherical and dense Ni_(y)Co_(z)(OH)₂, a lithium salt and anM-containing salt, they are heated and pulverized to obtain a cathodematerial for use in the lithium secondary battery represented by thechemical formula Li_(x)Ni_(y)CO_(z)M_(m)O₂ in which it is preferred toset the heating conditions to conduct a preliminary heating at atemperature of 300 to 500° C. for 2 to 6 hrs in an oxygen atmosphere, atemperature elevation process at 5 to 30° C./min after the preliminaryheating and a final heating process at a temperature of 650 to 900° C.for 2 to 30 hrs succeeding the temperature elevation process. M is oneor more of elements selected from Ba, Sr and B. Since the preliminaryheating is applied with an aim of completely removing the water contentof the water of crystallization in the starting material whilesuppressing reactions between lithium and nickel, it is preferred tokeep the temperature at 300 to 500° C. for 2 hrs or more. Since it wasconfirmed that water can be removed thoroughly at 6 hrs or less, it isnot necessary that the process exceeds 6 hrs in view of productivity.The temperature elevation rate in the temperature elevation process ispreferably 5 to 30° C./min in view of the protection of the heatingcrucible or refractory for heating and in view of productivity. Further,the temperature for the final heating is preferably from 650° C. to 900°C. The reaction proceeds slowly at a temperature lower than 650° C.,whereas scattering of lithium is caused at a temperature in excess of900° C. The retention time is preferably from 2 to 30 hrs in view of thereactivity and the productivity.

EXAMPLE

Cathode materials having the composition of the general formulaLi_(x)Ni_(y)CO_(z)M_(m)O₂ shown in Table 2 were prepared and specificsurface area, tap density, discharging capacity and DTG increase starttemperature thereof were measured. Finely powderous reagents were usedas the starting material for Nos. 11, 12, 15 and 16. Dense sphericalNi_(y)Co_(z)(OH)₂ with the tap density of 1.9 to 2.1 g/cm³ was used forother sample numbers as starting materials. As the heating condition,short time heating or long time heating described below in an oxygenatmosphere was adopted. The resUlt of measurement is shown in Table 2.

Short Time Heating Preliminary heating kept at 400° C. for 4 hrsTemperature elevation rate 10° C./min Final heating kept at 800° C. for4 hrs Long Time Heating Preliminary heating kept at 500° C. for 6 hrsTemperature elevation rate 20° C./min Final heating kept at 750° C. for12 hrs The specific surface area was measured by the so-called “BETmethod.”

TABLE 2 Ni—Co hydrates Average Specific Discharge DTG increase Tapdensity particle diameter Composition surface area Tap density capacitystart temperature No. (g/cm³) (μm) Li_(x)Ni_(y)Co_(z)M_(m)O₂ Heatingtime (m²/g) (g/cm³) (mAh/g) (° C.) 1 2.0 11.3Li_(1.02)Ni_(0.9)Co_(0.1)Ba_(0.001)O₂ Short time 0.42 2.1 194 219 2 1.911.8 Li_(1.02)Ni_(0.7)Co_(0.3)Ba_(0.001)O₂ Short time 0.41 2.0 180 230 32.1 10.2 Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ Short time 0.39 2.1 186227 4 2.1 10.2 Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ Long time 0.37 2.1185 229 5 2.1 10.2 Li_(0.96)Ni_(0.8)Co_(0.2)Sr_(0.007)O₂ Short time 0.452.1 181 230 6 2.1 10.2 Li_(0.99)Ni_(0.8)Co_(0.2)Ba_(0.002)Sr_(0.001)O₂Short time 0.41 2.1 185 230 7 2.1 10.2Li_(1.06)Ni_(0.8)Co_(0.2)B_(0.02)O₂ Short time 0.37 2.2 190 223 8 2.110.2 Li_(1.01)Ni_(0.8)Co_(0.2)Sr_(0.002)O₂ Short time 0.44 2.1 184 224 92.1 10.2 Li_(1.02)Ni_(0.7)Co_(0.3)Ba_(0.005)O₂ Long time 0.32 2.2 184248 10 2.1 10.2 Li_(1.06)Ni_(0.8)Co_(0.2)Sr_(0.005)O₂ Longtime 0.78 2.1185 225 11 1.6 27.2 Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ Short time1.21 1.4 177 201 12 1.7 25.6 Li_(1.02)Ni_(0.8)Co_(0.2)Ba_(0.001)O₂ Longtime 1.14 1.5 175 203 13 2.1 10.2 Li_(0.96)Ni_(0.8)Co_(0.2)Ba_(0.03)O₂Short time 0.58 1.7 150 220 14 2.1 10.2 Li_(0.96)Ni_(0.8)Co_(0.2)O₂Short time 0.38 2.1 190 190 15 1.6 27.2Li_(1.02)Ni_(0.7)Co_(0.3)Ba_(0.005)O₂ Long time 0.85 1.5 169 203 16 1.627.2 Li_(1.06)Ni_(0.8)Co_(0.2)Sr_(0.001)O₂ Long time 0.92 1.5 170 207

As shown in the table, the cathode materials having the composition ofthe invention and having the determined specific surface area have largedischarging capacity and high DTG increase start temperature. Further,when the tap density is large, the amount of the cathode material thatcan be filled in the battery can be increased and thecharging/discharging capacity per unit volume of the battery can beincreased.

While this invention has been described with reference to severalembodiments, it should be construed that such embodiments are only forthe purpose of specifically explaining the invention and do not restrictthe invention. After studying the present specification, variousmodifications equivalent materials or substitution of technique willoccur to those skilled in the art. All such modifications andsubstitutions are considered to fall within the scope of the presentclaims.

1. A cathode material for use in a lithium secondary battery comprisinga compound represented by the chemical formula:Li_(x)Ni_(y)Co_(z)M_(m)O₂ when not in a charged state, having highthermal stability and large discharging capacity in which M is selectedfrom the group consisting of Ba and Ba with Sr, and x, y, z and m arevalues for the molar ratio of each element: 0.9≦x≦1.1, 0.5≦y≦0.95,0.05≦z≦0.5 and 0.0005≦m≦0.005, wherein the compound represented by thechemical formula: Li_(x)Ni_(y)Co_(z)M_(m)O₂ when not in a charged statechanges to a compound when in a charged state represented by thechemical formula: Li_(a)Ni_(b)Co_(c)M_(n)O₂ and a, b, c and n are valuesfor the molar ratio of each element and are, respectively, 0.2≦a≦0.4,0.5≦b≦0.95, 0.05≦c≦0.5, and 0.0005≦n≦0.005 and the material is powdery,has a BET specific surface area 0.8 m²/g or less and has a DTG increasestart temperature at 215° C. or higher.
 2. The cathode material definedin claim 1, wherein the BET specific surface area is less than 0.5 m²/g.3. The cathode material defined in claim 1, wherein the compoundrepresented by the chemical formula: Li_(x)Ni_(y)Co_(z)M_(m)O₂ has a tapdensity of 1.5 g/cm³ or more.
 4. A lithium secondary battery containingthe cathode material defined in claim
 1. 5. A method of manufacturing acathode material comprising a powdery compound represented by thechemical formula: Li_(x)Ni_(y)Co_(z)M_(m)O₂, which M is Ba or Ba withSr, and x, y, z and m are values for the molar ratio of each element:0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5 and 0.0005≦m≦0.02, having a BETspecific surface area 0.8 m²/g or less, comprising: mixing only by a drymixing process a powder of a compound represented by Ni_(y)Co_(z)(OH)₂having a tap density 1.8 g/cm³ or more and an average particle diameterof 5 to 20 μm, a lithium salt and a salt containing the element M toform a dry mixture, and forming the cathode material by heating andpulverizing the diy mixture.
 6. The method defined in claim 5, whereinpowdery particles of the compound represented by Ni_(y)Co_(z)(OH)₂ havea spherical shape.
 7. The method defined in claim 5, wherein heatingincludes conducting successively preliminary heating at 300 to 500° C.for 2 to 6 hrs in an oxygen atmosphere, a temperature elevation processof elevating the temperature at a rate of 5 to 30C/min after thepreliminary heating and final heating at 650 to 900° C. for 2 to 30 hrssucceeding temperature elevation.
 8. A cathode material for use in alithium secondary battery comprising a compound represented by thechemical formula: Li_(x)Ni_(y)Co_(z)M_(m)O₂ having high thermalstability and large discharging capacity in which M is Ba or Ba with Sr,and x, y, z and m are values for the molar ratio of eachelement0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5 and 0.0005≦m≦0.005, wherein thematerial is powdery, has a BET specific surface area 0.8 m²/g or less,and has a DTG increase start temperature of 230° C. or higher in acharged state.
 9. The cathode material defined in claim 8, wherein theBET specific surface area is less than 0.5 m²/g.
 10. The cathodematerial defined in claim 8, wherein the compound represented by thechemical formula: Li_(x)Ni_(y)Co_(z)M_(m)O₂ has a tap density of 1.5g/cm³ or more.
 11. A lithium secondary battery containing the cathodematerial defined in claim 8.